Length of a Pendulum

A 319g ball is tied to a string. It is pulled to an angle of 7.60 degrees and released to swing as a pendulum. A student with a stopwatch finds that 14 oscillations take 13.0s.
What is the length of the string?

2. Relevant equations
I have already tried using T = 2pi sqrtL/g, but that does not give me the right answer.

Staff: Mentor

13 oscillations/14 seconds = 1.07 for T, and 1/1.07 = 0.928s for f.
Then I tried T = 2pi sqrt L/g, so 1.07 = 6.28 sqrt L/9.8m/s, then divided each side by 6.28 and got 0.170 = sqrt L/9.8, then squared each side, so 0.029 = L/9.8, then solve for L = 0.2842m, which is wrong according to MasteringPhysics.